.. code-block:: python

	#!/usr/bin/env python
	
	from __future__ import print_function
	import pymzml
	import os
	from operator import itemgetter
	
	def main():
	    """
	    Extract the 10 most often fragmented precursors from the BSA example file.
	
	    This can e.g. be used for defining exclusion lists for further MS runs.
	
	    usage:
	
	        ./get_precursors.py
	
	    """
	
	    example_file = os.path.join(
	        os.path.dirname(__file__),
	        os.pardir,
	        'tests',
	        'data',
	        'BSA1.mzML.gz'
	    )
	    run = pymzml.run.Reader(example_file)
	    fragmented_precursors = {}
	    for spectrum in run:
	        if spectrum.ms_level == 2:
	            selected_precursors = spectrum.selected_precursors
	            if spectrum.selected_precursors is not None:
	                for precursor_dict in selected_precursors:
	                    precursor_mz = precursor_dict['mz']
	                    precursor_i = precursor_dict['i']
	                    rounded_precursor_mz = round(precursor_mz,3)
	                    if rounded_precursor_mz not in fragmented_precursors.keys():
	                        fragmented_precursors[rounded_precursor_mz] = []
	                    fragmented_precursors[rounded_precursor_mz].append(spectrum.ID)
	
	    precursor_info_list = []
	    for rounded_precursor_mz, spectra_list in fragmented_precursors.items():
	        precursor_info_list.append(
	            (
	                len(spectra_list),
	                rounded_precursor_mz,
	                spectra_list
	            )
	        )
	
	
	    for pos, (number_of_spectra, rounded_precursor_mz, spectra_list) in enumerate(sorted(precursor_info_list, reverse=True)):
	        print(
	            'Found precursor: {0} in spectra: {1}'.format(
	                rounded_precursor_mz,
	                spectra_list
	            )
	        )
	        if pos > 8:
	            break
	
	if __name__ == '__main__':
	    main()
